Encryption is the process by which meaningful data, such as a file, is transformed into meaningless data that cannot be read without decrypting the data back into its meaningful form. In the vast majority of applications in which encryption is used, only one or very few people (or computers) have the knowledge and ability to decrypt the encrypted data and make it useful again. An encrypted file or data stream looks like a jumble of random letters, numbers, and other characters, and it is impossible to make sense of encrypted data without transforming it back into its original form. To perform this transformation, we need to know the cipher that was used to encrypt the data. A cipher is a set of logical instructions that can be followed in the same way every time to produce the same result. During the process of encryption, the instructions in the cipher are performed on the data to be encrypted. These steps change the readable data, or plaintext, into its unreadable, encrypted form, or ciphertext. Some ciphers also need a key in order to work on data. A key is a small piece of data that is used to customize each cipher. Each time a particular cipher is used, the key is changed. This allows a cipher to be used more than once without compromising its integrity. If a particular cipher was widely used and could be decrypted without a key, it wouldn't be secure. Anyone would be able to decrypt any data encrypted with this cipher. If keys didn't exist, a person would have to write a new cipher every time he or she wanted to encrypt a piece of data! It is very easy to generate a key, but very difficult to write a secure, effective cipher.
Encryption has been in use for thousands of years. The earliest known encryption was found engraved on Egyptian monuments from around 2500 B.C. Hebrew scholars used simple alphabetic substitutions, in which one letter of the alphabet stands for one other letter, with no letters standing for more than one and no letters being omitted. The ancient Greeks used encryption to transmit military messages. Encryption was used heavily during World War II to obscure radio transmissions and telegrams.